When an engine works, temperature of the cylinder cooling fin is very high, while temperature of the lower part of the casing is a little bit lower. A traditional portable engine adopts a design of an integrated air duct, which cools inner parts with cooling air drawn in by a fan and then expels through a vent groove. In such a way, direction and flow of cooling air cannot be well controlled, the cylinder head cover cannot be well cooled, and it is possible to heat the low-temperature part of the casing with hot air at the cylinder head cover.